Conventional processes for producing amino resin crosslinked particles are disclosed, for example, in Japanese Publication for Unexamined Patent Application Nos. 57091/1974 (Tokukaisho 49-57091) (published on Jun. 3, 1974), 45852/1975 (Tokukaisho 50-45852) (published on Apr. 24, 1975), and 211450/1992 (Tokukaihei 4-211450) (published on Aug. 3, 1992). The producing processes of amino resin crosslinked particles as taught by these publications proceed as follows. First, an amino compound is allowed to react with formaldehyde to prepare an amino resin precursor. The amino resin precursor is then mixed with an aqueous solution of an emulsifier to obtain an emulsion. Thereafter, a catalyst is added to the emulsion to cure the amino resin precursor in the emulsion state and obtain a suspension of amino resin particles. Subsequently, the amino resin particles are separated from the suspension and the cake containing the resulting amino resin is heated and dried to remove water from the cake and to complete cure (condensation) of the amino resin particles. One of the problems of the producing processes of the foregoing publications is that the amino resin crosslinked particles discolor. For example, the amino resin crosslinked particles may be discolored to yellow or other colors not originally intended. In the following explanations, the term “discoloring” will be used to indicate such a phenomenon in which the amino resin crosslinked particles are discolored to yellow or other colors not originally intended.
The amino resin crosslinked particles discolor because the amino resin particles are subjected to high temperatures when the cake is heated at a temperature above 100° C., for example, in a temperature range of 130° C. to 230° C., in a heating process that is provided to remove the moisture and unreacted formaldehyde contained in the cake of amino resin particles separated from the suspension, so as to improve the efficiency of condensation of the amino resin particles. It is therefore required to set a low heating temperature of, for example, not higher than 100° C. to prevent discoloring of the amino resin crosslinked particles.
A drawback of low temperature heating (e.g., 100° C. or below) in heating the amino resin particles is that heating takes an extended period of time and removal of the moisture and free formaldehyde becomes insufficient.
Another drawback is that condensation (crosslinking) of the amino resin particles becomes insufficient, with the result that hardness, heat resistance, and solvent resistance of the product amino resin crosslinked particles degrade.
It is therefore required, in order to improve hardness, heat resistance, and solvent resistance of the amino resin crosslinked particles, to heat the amino resin particles at a temperature above 100° C. (for example, 130° C. to 230° C.). Heating the amino resin particles at such a temperature causes the problem of discoloring on the amino resin crosslinked particles.
As the catalyst (curing catalyst) used to cure the amino resin precursor, various types of acid catalysts have been used conventionally. For example, the foregoing Tokukaisho 49-57091 and Tokukaisho 50-45852 use dodecylbenzenesulfonic acid (“DBS” hereinafter) as the catalyst. One drawback of DBS is its relatively slow reaction speed, which requires higher temperature heating or a process of long time heating. Another drawback of DBS is attributed to its color, brown, which causes the color of the amino resin particles to change in the heating process of the amino resin particles. In other cases, the DBS used as the catalyst may permeate into the amino resin particles to cause plasticization and to prevent stable crosslinking.
Further, Japanese Publication for Unexamined Patent Application No. 72015/1981 (Tokukaisho 56-72015) (published on Jun. 16, 1981) uses sulfuric acid as the catalyst. The sulfuric acid has no color and does not slow the reaction speed. However, specific examples of amino resin compositions described in this publication contain paratoluene sulfonamide (PTSA) as the amino compound. Containing PTSA as the amino compound is disadvantageous in the following respect. Namely, it discourages condensation because of small numbers of crosslinking sites present between the amino compound and formaldehyde. The product amino resin crosslinked particles, as a result, have insufficient hardness and insufficient heat resistance. That is, paratoluene sulfonamide is not preferable as the amino compound where high levels of hardness and heat resistance are required for the amino resin crosslinked particles. Further, the foregoing Japanese koukai publication does not describe neutralization of acid catalyst. The conventional techniques therefore fail to produce amino resin crosslinked particles with sufficient hardness and heat resistance, and without causing discoloring upon heating.